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Post subject: hum with master volume turned up on Blues Junior
Posted: Thu Jun 18, 2009 1:54 pm
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Hi - I'm not familiar with the technical aspects of a tube amp, but a few days ago my blues junior began to hum when the master volume was turned up high and the regular volume turned low. When I turn the master volume down, the hum progressively goes away. Now I know this can be a bunch of different things, but is there any easy way I can check the tubes (and which tubes in this case) to see if they are the problem? The hum is present even if there is no instrument cable plugged into the amp. Also, if I turn up the regular volume (while keeping the master down), there is no hum present.

Thanks for all the help ahead of time.
Jorge


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Posted: Thu Jun 18, 2009 2:14 pm
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You are cranking the preamp gain and a little hum or noise is normal. I be it has always done this. If not, you may just need a new preamp tube, which are the small tubes

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Post subject:
Posted: Thu Jun 18, 2009 2:24 pm
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So there are 4 12AX7 tubes, 3 are preamp and 1 is phase inversion. How can I tell which ones are for the preamp? And, do I need to have a tech rebias the amp, or can I just do a tube change with the same specs?


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Posted: Fri Jun 19, 2009 6:10 am
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Aspiring Musician
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I am not sure on the Blues Jr. Usually, the first preamp tube is the one closest to your guitar input. You might just try to replace all the 12ax7's. Try to get some Telefunkens if you can.

No bias is required for preamp tubes, just pull em out and replace. Do be carefull. Gently move the tubes from side to side as you remove them, don't force anything. Do the same as you install. This may or may not help the issue as most tube amps hiss when you crank the preamp gain

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Guitars:
1. mid 70's Ventura MIJ Gibson l6-s clone (Pre-lawsuit) in Black with Rosewood FB and EMG 81 BR/ 85 Neck
2. ESP KH2 Neck Thru
3. 2008 Am Std Strat in 3 tone sunburst


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